The advantages of cross-linking polymers by either chemical or irradiation techniques are well known. Cross-linking processes are used extensively in such fields as electrical insulation, foam and other industries. Some polyolefins such as polyethylene are especially suitable for chemical and irradiation cross-linking. However, polypropylene is prone to chain scission when subjected to high energy radiation, and as a result severe deterioration of the mechanical properties of irradiated polypropylene becomes a substantial problem. As a consequence, few industrial applications involving irradiation of polypropylene are found.
One significant application of industrial radiation sources is the sterilization of disposable medical devices such as sutures, syringes, bandages, labware and the like. The use of gamma rays from cobalt-60 sources is common although electron beam irradiation is also utilized. Polypropylene is used in some of the medical devices, and thus in sterilization applications. One significant problem still persists, namely the deterioration of polypropylene with time after radiation sterilization of the molded material.
U.S. Pat. Nos. 4,366,296 and 4,680,318, both assigned to Mitsubishi Petrochemical Company, disclose copolymers of propylene and branched 1,4-dienes such as 4-methyl-1,4-hexadiene or 5-methyl-1,4-hexadiene. U.S. Pat. No. 4,680,318 is specifically directed to foaming resin compositions and a method of producing radiation cross-linked foams. In both the '296 and '318 patents, the branched 1,4-diene formula used to define the dienes copolymerized with the propylene and ethylene specifically excludes 1,4-hexadiene. In "Comparative Example C1" of U.S. Pat. No. 4,366,296, an example is given of the copolymerization of propylene and cis-1,4-hexadiene, but it is stated that (1) it is difficult to control molecular weight with the addition of hydrogen to the polymerization reactants, (2) cis-1,4-hexadiene reduces the catalyst activity and (3) the heptane insolubles and bulk density of the copolymer are substantially lowered when cis-1,4-hexadiene is copolymerized with propylene.